This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying regions of rock formations from which hydrocarbons may be produced.
Hydrocarbon saturation S.sub.o is generally determined from water saturation S.sub.w as follows: EQU S.sub.o =1-S.sub.w. (1)
Water saturation present in a subterranean formation is typically determined from interpretation of conventional electrical (i.e., resistivity) logs recorded in a borehole drilled through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation set forth in "The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics", Trans. AIME, Vol. 46, pp. 54-62, 1942, by G. E. Archie. This equation is expressed as follows: EQU S.sub.w.sup.n =R.sub.w /.phi..sup.m R.sub.t, (2)
where S.sub.w is the fractional water saturation (i.e. free and bound water of the formation expressed as a percent of the available pore space of the formation, R.sub.w is the formation water resistivity, .phi. is the formation porosity, R.sub.t is the formation electrical resistivity, n is the saturation exponent and m is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict fractional water saturation S.sub.w from the formation resistivity, R.sub.t, using the equation in any of its forms.
Archie defined two quantities that provided the basis for his water saturation equation (1). The first quantity is the formation factor F which defines the effect of the rock matrix on the resistivity of water as follows: EQU F=R.sub.o /R.sub.w, (3)
where
R.sub.o =resistivity of water saturated rock and PA0 R.sub.w =water resistivity.
Archie reasoned that for a given value R.sub.w, the formation factor F would decrease with increasing porosity, .phi., to some exponent m: EQU F=1/.phi..sup.m. (4)
This porosity exponent m has also become known as the Archie cementation exponent. Thus Archie provided a useful characterization of a rock fully saturated with a conducting brine in terms of the water resistivity R.sub.w, porosity .phi., and a rock parameter m. It is important to note that Archie assumed all conductance to be in the brine.
The second quantity is the resistivity index I defined as the ratio of the resistivity of a rock partially saturated with water and hydrocarbon, R.sub.t, to the same rock saturated fully with water, R.sub.o, as follows: EQU I=R.sub.t /R.sub.o. (5)
Archie reasoned that as the water saturation decreased (i.e. hydrocarbon saturation increased) the resistivity R.sub.t and hence I would increase to some exponent n: EQU I=1/S.sub.w.sup.n ( 6)
where S.sub.w =volume of water in pores/total pore volume. This exponent n has become known as the Archie saturation exponent. It is again important to note that Archie assumed all conductance to be in the brine and further that all pores within the rock have the same water saturation S.sub.w.
It is these two equations (4) and (6) for the formation factor F and resistivity index I respectively that Archie combined to provide the water saturation expression S.sub.w of equation (2). Certain logs have provided formation resistivity R.sub.t and porosity .phi.. Water samples provide the best values for R.sub.w. Standard practice is to measure rock sample resistivities R.sub.o and R.sub.t for a number of water saturations and to plot the logarithm of I versus the logarithm of S.sub.w. Archie's equations assume such a logarithmic plot can be fit by a straight line with slope of -n.
Many core samples are, however, not homogenous and electrically isotropic. For such core samples, the Archie saturation exponent n will be strongly dependent on the direction the resistivity measurement is made. For example, a saturation exponent measured across permeability barriers within a core sample may be one and a half times as large as if it were measured parallel to the permeability barriers. This difference can have a large detrimental effect on the determination of hydrocarbon reserves derived from the calculated water saturation of equation (2). It is, therefore, an object of the present invention to determine resistivity of a core sample that is electrically anisotropic and to identify the degree of anisotropy changes as the brine saturation of the core sample changes so that an accurate water saturation can be calculated from equation (2).